Stringers are used as stiffening members. Examples of the use of stringers include stiffening the skin or cover of an aerofoil box, fuselage section or similar structure. Stringers may have a cross-section in the form of a T-shape, in the form of an L-shape or other suitable shapes. Typically, the stringer will have a foot which has a shape adapted to abut the surface of the structure to be stiffened and a web that projects from the foot away from the surface of the structure, the web increasing the stiffness of the stringer. The web is sometimes referred to as the blade of the stringer.
The thickness or the geometry of the surface of the structure to be stiffened may vary thereby producing local features in the face of the structure adjacent to the stringer. Thus, corresponding variations in the geometry of the stringer may be necessary. Local variations in the geometry of the stringer can however introduce manufacturing problems when fabricating composite stringers. For example, in order to increase the local strength or stiffness of an aircraft wing panel it is common practice to vary the thickness of the panel locally where extra stiffness or strength is required. This results in pad-ups in the panel profile in the stringer-facing surface. Thus, the thickness of the panel may, with increasing direction along the length of the associated stringer, ramp up to a locally thicker section and then ramp down to a thinner section. To accommodate the change in thickness in the panel, the foot of the associated stringer needs to correspondingly ramp up and ramp down. The shape of the stringer may therefore include local variations in its cross-sectional geometry, as a function of distance along its length.
The desired shape of stringer for use when stiffening a panel may therefore be complicated and may deviate from a linearly symmetrical geometry. Manufacturing composite stringers having a complicated geometry can be difficult. If local changes in cross-sectional geometry of the stringer are required, defects may be introduced during the manufacturing process. Such defects usually result from layers of fibre material being compressed or folded in regions where there is too much material in view of the local geometry. This can produce creases in the final product, typically in the form of transverse creases. Defects can also result from layers of fibre material being stretched and/or stressed in regions where there is too little material in view of the local geometry. This too can produce creases in the final product, typically in the form of longitudinal creases. Either of the foregoing types of defect (too little material or too much material) can result in undesirable weakening of, and/or localised internal stresses in, the composite material in such regions. Such defects are typically allowed for and suitable margins built in by adding extra material in such regions, to counteract the strength-reducing defects. Whilst the strength of the resulting component may not be thus compromised, this technique introduces a weight penalty and excessive structural volume.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved shape of composite stringer and/or an improved method of designing and/or manufacturing the same.